The invention relates generally to an apparatus and method for controlling auxiliary loads of a diesel engine, and more specifically to an apparatus and method for controlling the activation of auxiliary loads during the time immediately following a command to the diesel engine to provide additional output horsepower and prior to the time when the diesel engine has reached steady-state operation in response to that command.
Large self-propelled vehicles, such as locomotives, off-highway vehicles and transit cars commonly use a diesel engine to drive an electrical generating system, which in turn supplies electrical current to a plurality of direct-current (DC) or alternating current (AC) traction motors having rotors drivingly coupled, through speed-reducing gearing, to axle-wheel sets of the vehicle. The generating device typically comprises a main three-phase traction alternator, having a rotor mechanically coupled to the output shaft of the engine, which is conventionally a 16-cylinder turbo-charged diesel engine. When current excitation is applied to the field windings of the rotating rotor, alternating voltages are generated in the three-phase stator windings. These voltages are rectified and applied to the armature windings of the traction motors via a DC link. Typically, there is also an auxiliary alternator, which is also mechanically coupled to the output shaft of the engine, for producing alternating current to drive a plurality of auxiliary vehicle systems, such as a cooling radiator fan and a cooling traction motor fan.
During the motoring or propulsion operational mode, a locomotive diesel engine tends to deliver constant power from the traction alternator to the traction motors, as determined by the throttle setting and ambient conditions, but regardless of the locomotive speed. For maximally efficient performance, the electrical power output of the traction alternator must be suitably regulated so that the full engine power is efficiently utilized. For proper train handling, intermediate power output levels are provided to permit graduated power application to the traction motors, by controlling the excitation current supplied to the main alternator, through the operation of the operator-controlled handle or throttle, discussed further below. The traction alternator load on the engine must not exceed the power the engine is capable of developing. Engine overloads can cause premature wear, engine stalling or other undesirable effects. Historically, the locomotive control system has included the operator controlled handle, allowing the operator to select the traction power level, in discrete steps between zero and maximum, so that the traction and auxiliary alternators can supply the power demanded by the traction load and the auxiliary loads, respectively.
The engine horsepower is proportional to the product of the angular velocity of the crank shaft and the torque opposing crank shaft motion. To vary and regulate the engine power output, it is common practice to equip a locomotive engine with a speed regulating governor for adjusting the quantity of pressurized diesel fuel injected into each of the engine cylinders so that the actual crank shaft speed (in RPM) corresponds to the desired engine speed. The desired speed is set within permissible limits, by the lever or throttle handle that can be selectively moved through eight steps or notches between a low engine speed position (notch one) and a maximum engine speed (notch eight). The throttle handle is one element of the operator""s control console located in the cab of the locomotive. In addition to the eight conventional power notches, the handle further includes an idle position and a continuously variable dynamic braking position, allowing application of the dynamic brakes from zero percent to 100 percent of full allowable dynamic braking. The notch call or throttle handle position defines the engine speed and the engine load, as requested by the locomotive operator. A change from one notch position to the next consecutive notch position changes the delivered horsepower; certain notch position changes also command a change in the engine speed. In response to the throttle position the main locomotive controller commands the traction alternator to supply the demanded load, typically measured in the product of the traction alternator output current and the output voltage. The locomotive controller also responds to the engine speed demand at the notch position by controlling the fuel mass injected into each engine cylinder.
For each of its eight different notch settings, the engine is capable of developing a corresponding constant horsepower, assuming maximum output torque. The throttle notch eight position commands a maximum engine speed (e.g. 1,050 RPM) and a maximum rated gross horse power (e.g. 4,500). The engine output power at each notch position is equal to the power demanded by the traction motors, as supplied by the engine-driven traction alternator, plus the power demanded by the electrically driven auxiliary equipment or loads. Each notch position commands a different engine load or horsepower, but a few of the notch positions command the same engine speed with different horsepower values.
The output power (measured in kVA) of the main or traction alternator is proportional to the product of the RMS magnitude of the generated voltage and load current. The voltage magnitude varies with the engine speed and is also a function of the excitation current supplied to the alternator field windings. To accurately control and regulate the power supplied to the electrical loads (i.e., the main traction motors and the auxiliary loads), it is common practice to adjust the field or excitation current supplied to the main alternator to compensate for load changes, i.e., changes in the traction motor loading and/or auxiliary equipment loading. This minimizes the error between the actual output power and the desired output power and reduces the engine load. The desired output power is established by the locomotive operator by placement of the throttle handle in one of the notch positions one through eight. The resulting control over the excitation current creates a balanced steady-state condition resulting in substantially constant and optimum electrical power output for each position of the throttle handle.
It is also desirable to control the engine fuel flow to maintain a constant engine RPM for the notch position horsepower. Sudden changes in demanded horsepower (either by way of the traction or the auxiliary alternator) can cause the engine to be temporarily over-fueled or under-fueled due to the compensation made by the controller to maintain engine speed. If the engine is over fueled, the resulting low air-to-fuel ratio causes incomplete combustion, resulting in excessive exhaust emissions from unburned hydrocarbons. If insufficient fuel is supplied, the engine may bog and stall.
Recent amendments to the United States environmental protection statutes and regulations mandate specific visible smoke/particulate and invisible emissions levels from locomotive diesel engines. One such requirement is the reduction of oxides of nitrogen (NOx) emissions, which can be lowered by retarding the injection fuel timing of the diesel engine. But this timing modification increases fuel consumption and operating costs and therefore it is desirable to increase the engine compression ratio to gain back some of the fuel consumption losses. However, increasing the compression ratio increases the visible smoke emissions when the engine is not fully loaded. The problem of visible smoke is especially acute during low load conditions and transient load and speed changes, i.e., when the locomotive operator advances the throttle to a higher notch position to call for higher speed and/or greater load pulling capacity (i.e., horsepower). NOx emissions are especially prevalent at high engine loads.
Given the substantial focus on the reduction of smoke and NOx emissions, many different techniques for lowering these emissions have been proposed. One class of solutions involves the after-treatment of the exhaust stream by use of selective catalytic reductions (e.g. injecting urea into the exhaust system to reduce NOx emissions) and catalytic converters. A second solution involves in-cylinder treatment to reduce the formation of a particular pollutant within the combustion chamber. In-cylinder control can be achieved through the manipulation of a fuel injection parameter (such as injection pressure, spray angle, timing, etc.), exhaust gas recirculation, water emulsification or the direct injection of water into the combustion chamber. Attention has also been focused on new injection systems including common rail unit pumps, split injection and injection rate shaping.
The auxiliary alternator discussed above supplies power to the auxiliary loads that include the motors powering the radiator fan, the traction motor blowers and the alternator blower. The auxiliary alternator also powers the brake system air compressor, the battery charging system and the main (or traction) alternator excitation system. Typically, the radiator fan, the traction motor blower and the air compressor are powered by three-phase multi-speed AC motors that run synchronously with the engine revolutions. The availability of multiple speeds for these motors is achieved through the use of cycle skippers that control the frequency of the alternating current (AC) delivered to the blower motors. To increase the speed of an auxiliary motor, for example, from xc2xc speed to xc2xd speed (i.e., where the fraction is with respect to maximum engine speed), the cycle skipping process requires that the motor first be brought to full speed, the input power removed, allowing the motor to coast down to the desired speed. When the motor has reached the desired operating speed, power is reapplied, but at the proper AC input signal frequency to maintain the desired speed. For example, for operation at xc2xd speed, the AC frequency signal is divided by two. Typically, only the radiator cooling fan and the traction motor blower operate at less than synchronous speed. In particular, the cooling fan is operative at xc2xc, xc2xd and full speed and the traction motor blower is operative at xc2xd or full speed.
At lower notch positions, i.e., when the engine is delivering less energy, the horsepower required to start or increase the speed of an auxiliary load, for instance, the radiator fan, can be about the same order of magnitude as the steady-state traction motor load, thus creating a large load on the diesel engine. Analysis of the auxiliary load situation further must consider the possibility that additional auxiliary loads (e.g., blowers or fans) may be added to future locomotives to further reduce steady-state and transient emissions limits.
It is also known that when the locomotive is operating in the mid and lower notch positions, there is insufficient energy in the exhaust system to power the engine turbocharger and as result, the engine behaves as a naturally aspirated engine. Under these conditions, the engine operates with lower air-to-fuel ratios and is more sensitive to transient load changes, that is, either an increase in speed and/or load. If the speed and/or load is increased too rapidly, smoke and particulates are formed and the engine performance is degraded.
The increasingly stringent environmental regulations mentioned above suggest the development of a better strategy to control transient loading of the diesel engine to ensure compliance with applicable emission regulations. Current Environmental Protection Administration regulations permit short term visible emission spikes (of 5 seconds or less) during steady-state operation (defined as operation during which there is no commanded speed and/or load change) to allow air compressor operation to maintain the brake system reservoirs at full pressure. However, if an auxiliary load other than the air compressor is energized during a commanded speed or load change (a notch change) the emission limits must be met.
The present invention provides improved control over the auxiliary loads, i.e. over the auxiliary power demand of a diesel engine during notch or throttle position changes that command a higher engine output. During load increases, the engine demands more fuel. But, because the air handling system lags the delivery of the increased fuel, the optimum air-to-fuel ratio that provides complete combustion of the fuel is not maintained. Additional simultaneous load demands by an auxiliary component (e.g. the air brake system compressor, radiator fan or motor fan blowers) further increase the engine load, causing the formation of additional smoke and particulates. The auxiliary control system according to the present invention monitors, screens and prioritizes the application of additional auxiliary loads and when possible, defers the application until the load increase demand on the engine due to the throttle position change has been satisfied, i.e., the engine has reached steady-state operation at the new load value. The prioritization scheme is based on the operating condition of the engine and the specific auxiliary load requesting activation. Also, if operating conditions do not permit deferral of the additional auxiliary load, then the auxiliary loads are sequentially switched on and off to avoid a situation where several auxiliary loads simultaneously demand additional power from the diesel engine.